1. Field of the Invention
The invention relates to coating glazing material. In particular it relates to a coating which comprises several layers of material deposited in a defined and advantageous sequence upon the glazing.
The expression "glazing material" is used herein to denote sheets of plastics or vitreous material which are used in the performance of the glazier's art. "Vitreous material" includes glass and vitrocrystalline material. Such sheets are most often transparent and clear, but they may be coloured and/or they may be merely translucent or even opaque. As an example of an opaque glazing panel may be cited a panel for mounting beneath a transparent panel, for example in an interior partition, where it is desired to reproduce the texture of the upper transparent panel without allowing sight through such lower panel.
2. Description of the Related Art
The wide variety of uses for glazing sheets and panels has led to a considerable breadth of knowledge on coating the glazing so as to improve particular qualities. Coatings may for example be provided to protect the glazing surface, to colour the glazing or to create an electrically conducting layer across it.
In recent years considerable research effort has been put into the selection of multi-layer coatings which impart low emissivity and/or other optical or energy transmitting or reflecting properties to glazing panels for use in vehicles and buildings.
One aim has been to reduce heat loss from within the space enclosed by the panel, while neither preventing the admission of solar heat nor hindering a high level of light transmission in both directions. One previously proposed way of achieving this has been to provide low emissivity qualities by employing a coating made up of several thin layers of materials which complement each other to achieve the desired result.
Most of the required optical features can in principle be provided by a single coating of reflective metal, for example silver, applied as a layer sufficiently thin to permit transmission of most of the radiation in the visible portion of the spectrum while reflecting most of the infra red portion. If used alone however such a thin metal layer tarnishes in the atmosphere, giving discolouration, reduction of light transmission and a tendency to fragment. It is also of limited mechanical strength and thus prone to chipping, especially at the edge of the glazing panel, and to abrasion.
Other layers are therefore applied in combination with the reflective layer so as to protect it physically against abrasion and chemically against corrosion. These further layers must moreover be selected from materials which do not significantly weaken the optical properties of the coated glazing. The layers immediately adjacent the reflective layer are most commonly of metal oxides, sometimes in combination with other materials such as varnishes, plastic laminates or further sheets of glazing. Such adjacent layers are employed in some instances to improve the optical qualities by acting as a non-reflective layer for the visible portion of the spectrum.
One of the most commonly used coating materials is tin oxide, typically applied as a layer on both sides of the reflective metal layer. This provides many of the required qualities and is also generally inexpensive. It has good optical properties especially as a non-reflective layer (if applied to an appropriate thickness) and also binds well to the adjacent layers. It has been used both under the reflective metal and above it. There have also been several prior proposals to add to the tin oxide, or replace part of it, with a further metal or metal oxide to maintain particular chemical, physical or optical qualities of the coating as a whole. The choice of added materials, and the sequence in which they are applied to the glazing, is however a complex matter since there is a tendency for a material selected to improve one quality to diminish one or more of the others. This may in turn call for a further layer to correct the adverse effect on such other qualities.
A typical example of the complex layer structure that results is described and claimed in European patent specification EP-A-226993. This discloses a high transmittance low emissivity coating on a glass substrate which includes an oxide reaction product of an alloy containing zinc and tin as a first transparent anti-reflective film, copper as a primer film deposited on the first film, silver as a transparent infrared-reflective film deposited on the primer, an oxide reaction product of an alloy containing zinc and tin as a second transparent anti-reflective film deposited on the silver and titanium dioxide as a protective overcoat.
Similar coatings are described in European patent specification EP-A-104870 which discloses in example 1 a float glass pane coated in turn with a tin oxide layer, a silver layer, a copper layer and a further layer of tin oxide. Each of the tin oxide layers is 30-50 nm thick, the silver layer 8 to 12 nm and the copper layer just 1 to 5 nm.
European patent specification EP-A-275474 describes and claims a high transmittance, low emissivity heatable article comprising a transparent nonmetallic substrate, a first transparent anti-reflective metal oxide film comprising zinc deposited on a surface of said substrate, a transparent infrared reflective metallic film deposited on said anti-reflective metal oxide layer, a metal-containing primer layer deposited on said infrared reflective metallic film, wherein said metal is selected from the group consisting of titanium, zirconium, chromium, zinc tin alloy and mixtures thereof, and a second transparent anti-reflective metal oxide film comprising zinc deposited on said metal-containing primer film.
A proven technique for applying such layers is cathodic sputtering. This is conducted at very low pressures, typically of the order of 0.3 Pa, to give a layer of the coating material across the glazing surface. It can be conducted under inert conditions, for example in the presence of argon, but alternatively can be effected as reactive sputtering in the presence of a reactive gas such as oxygen.
European patent specification EP-A-183052 describes the use of reactive sputtering of a cathode target of an alloy of zinc and tin in an oxygen atmosphere so as to apply to a substrate of glazing material an oxide reaction product of the alloy.
European patent specification EP-A-219273, which is largely concerned with an electrically conductive coating for motor vehicle windows, describes a coating process (and the product thereof) in which a dereflecting layer such as zinc oxide is first deposited, followed by a transparent silver layer, a sacrificial metal layer (for example of titanium), a layer of titanium dioxide and a second dereflecting layer. In this process both the dereflecting layers are deposited by reactive sputtering.